(a) Fields of the Invention
The present invention relates to optical devices for reading images, particularly solid-state imaging devices and their fabrication methods, camera modules using the solid-state imaging devices, and electronic equipment mounting the camera module.
(b) Description of Related Art
A solid-state imaging device reads images in the manner in which a light pattern projected onto an imaging surface by a lens or the like is stored in the form of charges by a great number of photodiodes provided on the imaging surface, and this solid-state imaging device is often employed in the field of digital cameras, cellular telephones, and in addition cameras for endoscopes or the like. In such a solid-state imaging device, the photodiode area decreases with an increase in the number of pixels, which leads to degradation of sensitivity. Therefore, the device is required to enhance the sensitivity.
One of the most commonly used methods for enhancing the sensitivity is a method for increasing the aperture ratio of a photodiode in a pixel. Increasing the aperture ratio of the photodiode itself in the pixel is, however, difficult from the viewpoint of pattern design. From this, as a method for providing an effective increase in the aperture ratio of the photodiode, use is made of a method for increasing the amount of light incident to the photodiode, that is, a method for increasing the light collection efficiency thereof. This method is implemented so that a microlens having an area smaller than the area of the pixel and larger than the area of the photodiode is formed over the surface of each pixel.
However, the thickness of a photoresist necessary for microlens formation is 2 to 3 μm, and the limit of the pattern resolution of a space between microlenses is 0.8 to 1 μm. Therefore, as the pixel area decreases, the influence of the space area, which is an unnecessary area, becomes stronger. This leads to a situation in which the effective light collection ability of the microlens to the pixel area, that is, the aperture ratio thereof cannot be increased.
Moreover, as typified by cellular telephones and the like, electronic equipment using a camera module is also required to reduce its size and thickness. Thus, for the conventional package structure in which a solid-state imaging element is disposed in a ceramic package and sealing is performed by bonding a glass plate to the front surface, it has become impossible to satisfy the above requirement. From this circumstance, another package structure is also being developed which provides flip-chip packaging by directly attaching a glass plate onto a microlens array.
For example, as a first example, the following structure is proposed (see, for example, Japanese Unexamined Patent Publication No. H5-110960): in a solid-state imaging device in which a solid-state imaging element chip with a microlens formed on an imaging area is provided over a substrate, at least part of the perimeter of the solid-state imaging element chip except the imaging area is provided with a convex wall, a transparent member is disposed on the convex wall to face the microlens, and a sealing member surrounds an area ranging from the substrate to the transparent member to hermetically seal the microlens. Such a structure can seal the microlens to prevent degradation thereof by moisture and variation in refractive index.
As a second example, another structure is proposed (see, for example, Japanese Unexamined Patent Publication No. 2000-138361): the structure includes a solid-state imaging element chip, a color filter of an inorganic material or the like which is provided on the solid-state imaging element chip and which can withstand high temperatures above 200° C., on-chip microlenses each arranged on the color filter at a position corresponding to a light receiving part of the solid-state imaging element chip, and a protective layer of a relatively hard, transparent material provided to cover the on-chip microlens, and the surface of the protective layer is planarized. By thus providing the protective layer, a dedicated package becomes unnecessary and concurrently operations for individual chips carried out after dicing can be reduced to simplify the fabrication steps. Moreover, according to the second example, the protective layer is relatively hard and its surface is planarized. Therefore, even though dust adheres thereto, it can be wiped out easily without scratching.
As a third example, still another structure is proposed (see, for example, Japanese Unexamined Patent Publication No. H6-232379): in a solid-state imaging element chip having microlenses formed over an imaging area, unevenness created by the microlenses provided on the surface portion is planarized by transparent resin having a lower refractive index than the microlens, and the top of the transparent resin is formed with a transparent protective layer having a higher mechanical strength than the transparent resin. By such a structure, the surface of the solid-state imaging element chip is planarized and the mechanical strength thereof is enhanced. Therefore, according to the third example, contamination such as dust adhering to the surface of the microlens can be removed, by a cotton swab and the like, without breaking the microlens.
As a fourth example, yet another structure is proposed (see, for example, Japanese Unexamined Patent Publication No. H4-226073): in order to mainly increase the microlens aperture ratio, the surfaces of microlenses formed by a conventional method are formed with a microlens cover film with a uniform predetermined thickness to decrease the effective space between the microlenses. This method describes the fact that the microlens cover film is made of silicon oxide (SiO2), silicon nitride (Si3N4), silicon oxynitride (SiON), or the like. By thus stacking the microlens cover film on the surfaces of the microlenses, the diameters of the microlenses can substantially increase to decrease the effective space between the microlenses. This increases the effective aperture ratio in the pixel to improve the light sensitivity thereof.
In the first example described above, the device is constructed so that the solid-state imaging element chip is sandwiched between a ceramic base and a cover glass and the outer circumference thereof is sealed by sealing resin. Therefore, the sealing performance is improved, while it is difficult to reduce the thickness after packaging. Furthermore, the ceramic base is relatively expensive, which in turn causes a cost reduction problem.
In the second example described above, the surface of the relatively-hard protective layer covering the microlenses is planarized. Thereby, this process can be carried out in the state of the solid-state-imaging-element wafer formed with the multiple solid-state imaging element chips, and then the resulting wafer can be subjected to dicing. This results in cost reduction. However, in this example, the hard protective layer is formed directly on the microlenses. This example describes a concrete example in which SiO2 is formed by a CVD method. When a thick SiO2 film with a small thermal expansion coefficient is formed on the microlenses made of resin, stress from the protective layer may deform the microlenses to degrade the functionality as a lens. Moreover, it is difficult to form a thick protective layer. Thus, when dust or the like adheres onto the surface of the protective layer, the dust wiping applies pressure also to the microlenses to deform them and concurrently to cause cracks and the like in the protective layer. This will degrade the reliability.
In the third example described above, processing is made in the state of the solid-state-imaging-element wafer formed with the multiple solid-state imaging element chips. When the transparent protective layer with a high mechanical strength formed as the finally-provided surface is formed thick, it becomes difficult to remove a portion of the transparent protective layer located on the pad in order to establish external connection. Thus, the transparent protective layer cannot be formed thick, which causes the problem that adequate protection against the case where a mechanical force is applied by dust wiping or the like cannot be made.
In the fourth example described above, the microlens cover film also has an uneven contour similar to the microlens. Therefore, dust or the like adhering to the surface cannot be wiped, so that it becomes necessary to additionally cover the surface with a transparent member such as a cover glass. This makes it difficult to reduce its size and thickness.